U.S. Pat. No. 4,135,124 in the name of one of us, Robert Buck, discloses a monitoring system of the type here considered which includes logic circuitry for selectively presetting that system to respond to the occurrence of an ambient condition being sensed, i.e. to a change in the value of a binary signal resulting therefrom, with either an intensification or a reduction of the load current. This is accomplished with the aid of an Exclusive-OR (XOR) gate, of the regular or the negated type, which has a first input coupled to the output lead of the detector and a second input provided with switchover means for selectively applying two different voltages to the latter input, either manually or automatically . Depending on the logical value of the applied voltage, the XOR gate transmits either the unmodified binary signal or its logical complement to a control unit of an electronic switch such as a thyristor. Thus, the XOR gate acts as a selective signal inverter whereby, for example, a mechanical relay controlled by the electronic switch can either open or close a load circuit upon the occurrence of the event being monitored.
An improved system of the latter type, with selective signal inversion, is the subject matter of a copending application, U.S. Ser. No. 487,123, filed on even date herewith by two of us, Robert Buck and Jean Luc Lamarche. In one of the embodiments described in that copending application, the energy source in series with the load is a generator of direct current in contradistinction to the alternating-current source used in the systems of the above-identified Buck patent and other patents listed therein.
In these earlier systems, in which the alternating supply voltage is converted into a pulsating unipoler voltage by a fullwave rectifier downstream of the load, a variable-impedance network responsive to a changeover in the output signal of the detector advantageously comprises a thyristor or equivalent electronic device which is switched in a binary mode so as to let the load current vary between two predetermined magnitudes. This is desirable since the output signal of the detector, though itself regarded as of binary character, may in fact assume different values above and below a certain threshold which the load is intended to recognize. The prior thyristor circuits, however, cannot be employed with a d-c supply since a thyristor, once triggered by a positive-going gate voltage, remains conductive until its anode current is cut off. The simple substitution of a transistor for such a thyristor obviates this problem but does not establish a binary mode of operation. Moreover, care must be taken to insure that the detector, e.g. an oscillator working into a trigger amplifier, receives the necessary operating current in both the conductive and the nonconductive state of the electronic switch; when that switch is cut off, the current drawn by the detector must be low enough to prevent a premature tripping of the load (e.g. an electromagnetic relay). whereas conduction of that switch must not deprive the detector of is minimum voltage supply. In earlier monitoring systems utilizing an a-c source, this requirement has been satisfied with the aid of a current limiter, e.g. a constant current unit of the type described in prior U.S. Pat. No. 4,193,023 in the names of Robert Buck and Gerd Marhofer.